All the methods described in this module to reduce the carbon content of concrete focus on reducing the cement content in the concrete mix. The use of fly ash and slag are well established methods that have a proven track record and existing supply network. There are numerous other methods to reduce the cement content of concrete and this section will focus on just two of the emerging technologies.
Kaolinite
Certain naturally occurring clays, known as kaolin or china clays, which are traditionally used to manufacture porcelain can also be used to produce a cement like pozzolanic material. The key mineral in these clays is called kaolinite. Different clays possess a wide range of kaolinite content that would be a suitable pozzolanic material. A pozzolan is a broad class of both naturally and manmade materials that have cementitious properties when combined with water and lime (calcium hydroxide). Both fly ash and slag are considered manmade pozzolans. Kaolin clays can be heated up to remove most of the adsorbed water and become a highly pozzolanic material that acts as an excellent SCM for concrete applications. This thermal activation process is also known as calcining. Koalin clays naturally occur all over the world but some areas have a much higher concentration of these clays than others.
There’s a resurgence in this technology in recent years. Most notably, the efforts of the LC3, Limestone Calcined Clay Cement group, are starting to prove large scale production capactity of these clay based concrete in various locations in India, Cuba, and China (www.lc3.ch). The figure below shows that using calcined clay as an SCM can reduce cement content by 50%, drastically lowering the carbon footprint of the concrete mix without introducing industrial waste products such as fly ash and slag.
Recycled Glass
Another emerging pozzolanic material is recycled glass. A limited number of companies are starting to crush glass into a fine powder-like silica substance that is regarded as a natural pozzolan and already approved for use by ASTM 618 – “Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete”. Glass is made from sand, which is an abundant natural resource. While there are few examples to draw from with this emerging technology, there seems to be great potential for recycled glass to be used in this manner. Recycling glass bottle into glass bottles can already be challenging in certain areas of the country that don’t have strong glass recycling markets. Additionally, the cost to transport heavy glass to recycling markets can be cost prohibitive. Although there is little data to evaluate the concept of using recycled glass as a cement substitute, this use may be the highest and best use of recycled glass. Additionally, one company reports that their recycled glass SCM product is non-toxic and contains no crystalline silica or heavy metals.
Conclusion
Concrete is an abundantly used construction material that has substantial greenhouse gas impacts primarily caused by the production of cement. Fortunately, the concrete industry is already familiar with using supplementary cementitious materials (SCMs) such as fly ash and slag to reduce the cement content of concrete mixes. Reducing the cement content is the most effective strategy to lower the greenhouse gas impacts of concrete. In some markets, fly ash and slag are even less expensive than cement. One challenge to currently using fly ash and slag is its potential effects on the construction schedule. When used as a cement replacement over 30%, both fly ash and slag can have low early strength characteristics that may affect the project schedule if used for certain structural applications. Early construction planning and targeted specifications can help alleviate most of these drawbacks. Government purchasers have numerous strategies at their disposal for lowering the greenhouse gas impacts of concrete purchases. Overall, the concrete industry is actively developing and testing new supplementary cementitious materials to meet the demand for long lasting, low impact and safe concrete products. The strategies outlined in the concrete module of the Climate Friendly Purchasing Toolkit will allow government purchasers to make meaningful reductions in their greenhouse gas impacts of their concrete purchases.